Civil and military structures, such as buildings, dams, and bridges can benefit from smart sensors that can report the peak strain to which they have been exposed. Strain to such structures may peak when power is not available, such as during hurricanes, tornadoes, and earthquakes. Devices that do not need power that can measure and record the extent of strain that may have occurred at these times are especially important. Similarly, structures that are repeatedly stressed, such as helicopter and aircraft landing gear, and which may need to be repaired, reinforced, or replaced if strain exceeds a threshold, could benefit from strain monitoring, but these structures are difficult and expensive to permanently instrument by hard wires to a data recorder. Sensitive instruments may also be subject to rough handling during transport, but data recorders powered by batteries may not be a practical or cost effective monitoring solution for such instruments. Similarly, recording details of what happens in a vehicular collision or in a collision between athletes can be of substantial value, and advantage here is especially great for a device that does not require a source of power for making its measurement. In addition to sensors for peak displacement and strain detection, it is desirable to also provide sensors for peak acceleration, force, pressure, and torque.
Commonly assigned U.S. Pat. No. 5,777,467 to Arms, et al, (“the '467 patent”) incorporated herein by reference, describes a novel ultra-miniaturized differential variable reluctance transducer assembly encased in stainless steel. The assembly contains a free sliding, magnetically permeable core and two coils surrounding the core. A split ring mounting adapter system allows for a variable gauge length and interchangeable mounting pins. A highly flexible core carrier tube and support wire allows for significant bending without failure, does not interfere with the coils detection of the core, and protects the core from corrosion. A sleeve strain relief sheath has been incorporated with the sensor to avoid excessive strain to lead wires during and after installation. The position of the core is detected by measuring the coils' differential reluctance and transmitted by means of wires or telemetry to measuring equipment. However, while the '467 patent is suitable for differential strain detection, it does not provide for holding a peak strain reading.
Commonly assigned patent application Ser. No. 09/259,615 to Arms, et al, (“the '615 application”) incorporated herein by reference, describes a passive peak strain detector that is especially useful in circumstances where power is unavailable. The patent application demonstrated that tiny, peak strain detection devices with high strain resolution are fabricated using a differential variable reluctance transducer with an entrapment collar that provides a circumferential load to the core to constrain it from free sliding, holding the peak strain measurement for reading at a later time. No power is required for making the reading. However, tiny coils, such as those used for the variable reluctance sensors described in both the '467 patent and in the '615 application, have windings with delicate wires that are difficult to handle and difficult to interconnect with other circuitry.
In U.S. Pat. No. 4,759,120 to Bernstein (“the '120 patent”), discloses a coil wound around a core. Insulation is removed from the wire either during or after winding at predetermined locations to match the location of connection pads in a conductive pattern on a substrate. The coil is appropriately aligned and laid down on the substrate and an attachment technique is used to form an electrical connection between the exposed areas of wire and connection pads on the substrate. The '120 patent provides for varying the inductance of the coil so formed by controlling the location of openings in the insulation. However, the '120 patent provides no way of varying the inductance of the coil after mounting to the substrate.
Similarly, the spring loaded entrapment collar disclosed in the '615 application has no adjustment, and the very large force required to reset the device makes reuse impractical.
Thus, a better system for manufacturing, handling, and using coils and clamps is needed to provide lower cost variable reluctance sensors that can detect and hold a peak reading, and clamps that can be reset, and these solutions are provided by the following invention.